Question: Which of these numbers is prime? ${15,\ 25,\ 35,\ 36,\ 41}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 25 are 1, 5, and 25. The factors of 35 are 1, 5, 7, and 35. The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, and 36. The factors of 41 are 1 and 41. Thus, 41 is a prime number.